(a) Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device and, more particularly, to a method for manufacturing a semiconductor device having improved characteristics.
(b) Description of the Related Art
With the recent improvement of a semiconductor device in integration and operational speed, transistor dimensions are remarkably reduced, which in turn focuses the difficulty in obtaining a high reliability for the transistors.
Patent Publication No. JP-A-1985-97628, for example, proposes a first technique for fabricating a semiconductor device in which a silicon nitride film overlying an active layer for a transistor is deposited by a plasma enhanced chemical vapor deposition (PECVD). The silicon nitride film provides therefrom a large amount of active hydrogen ions to the silicon active layer during a heat treatment of overlying layers, the active hydrogen ions providing passivation of dangling bonds in the silicon substrate to thereby recover the transistor characteristics damaged by a plasma or ion-implantation step.
The semiconductor device having the PECVD silicon nitride film as described above, however, suffers from a low durability against a hot carrier problem. The reason therefor is that a heat treatment for the PECVD silicon nitride film produces defects caused by desorption of hydrogen in the silicon nitride film, and water (moisture) or hydrogen ions will diffuse through the defects into the active layer.
Patent Publication No. JP-A-1983-129333 proposes a second technique to solve the hot carrier problem, in which a first silicon nitride film overlying an active layer is formed by a low-pressure CVD (LPCVD) technique and a second silicon nitride film is formed by a PECVD technique, whereby hydrogen ions from the PECVD nitride film are stopped by LPCVD silicon nitride film and provided through a metallic plug, which is connected to the active layer, into the active layer to provide passivation of the dangling bonds.
Patent Publication No. JP-A-1992-188675 proposes a third technique to solve the hot carrier problem, in which a first silicon nitride film overlying an active layer is deposited by a LPCVD technique, and a second silicon nitride film is deposited by a PECVD technique as a final layer for stopping the hydrogen ions.
FIGS. 1A to 1D show consecutive steps for the third conventional method as an example for solving the hot carrier problem. In FIG. 1A, a gate insulating film 12 and a gate polycrystalline silicon (polysilicon) film 13 are consecutively formed on a silicon substrate 11, followed by introduction of impurity ions by using the gate structure as a mask for forming lightly doped diffused regions in the silicon substrate 11. Subsequently, side walls 15 are formed on the gate structure and highly doped diffused regions 14 for source and drain are formed by ion-implantation using the side walls 15 as an additional mask. The gate polysilicon film 13 and heavily doped regions 14 are electrically separated from each other by the side walls 15.
Subsequently, as shown in FIG. 1B, a silicon oxide film 16 is formed on the entire surface, and a first silicon nitride film 17 is then deposited thereon, as shown in FIG. 1C, by a LPCVD technique. Thereafter, an interlayer insulating film 22 is formed on the first silicon nitride film 17, followed by deposition of a second silicon nitride film 23 by a PECVD technique.
In the second and third conventional methods, hot carrier problem can be solved to some extent. However, transistor characteristics are poor due to plasma damage etc. Specifically, Ti etc. in the metallic plug in the second method will absorbs hydrogen so that an adequate amount of active hydrogen ions is not provided to the active layer in the substrate, although the adequate amount of active hydrogen ions is necessary for passivation of dangling bonds in the silicon substrate to recover the silicon surface from a plasma damage etc. The first silicon nitride film 17 in the third method also prevents an adequate amount of active hydrogen ions from being diffused from the second silicon nitride film 23 to the active layer 14 in the silicon substrate 11.